The toxicity of produced formation water (PFW) originating from four natural gas production platforms located in the Adriatic Sea (Italy), and of seawater samples collected near these installations is assessed by means of bioassays with Brachionus plicatilis, with the brine shrimp Artemia franciscana and with Paracentrotus lividus. The toxicological response of these specimens was evaluated in order to identify the most sensitive one, in the consumer compartment, and to design a bioassay battery specific for samples collected in the surroundings of a gas platform. Larval mortality of rotifers, larval immobilization of crustaceans, fertilization success/failure (sperm cell test) and larval (pluteus) development success/failure (embryo toxicity test) of sea urchins were taken as ecotoxicological endpoints. The PFW sampled on two platforms resulted toxic, while no toxicity was recorded in seawater samples collected in the vicinity of the platforms, even in coincidence with the PFW discharge operations. The species and bioassays employed have shown different responses to PFW: P. lividus turned out to be more sensitive than A. franciscana and B. plicatilis. In particular, the embryo toxicity test showed a higher toxicity than the sperm cell test.